This is an investigation of flow disturbances at vascular anastomoses and their relationship to anastomotic hyperplasia. Flow visualization studies using visual dyes in clear plastic models are performed under steady and pulsatile flow conditions. These studies have demonstrated that flow separation occurs in models of anastomoses commonly used in clinical vascular surgery. This results in contrasting regions of high shear and low shear in the anastomotic region. The location and extent of separation is highly dependent on the angle of anastomosis and the flow split between the two runoff branches, proximal and distal. Laser Doppler anemometry demonstrates dependency of the shear rate on these factors. In vivo models of dacron-arterial anastomoses were studied using femoro-femoral dacron bypass grafts in dogs. Hyperplasia at the anastomoses was measured using a digitized pad for integration of projected areas of microscopic sections. The primary location of the hyperplasia is at the distal anastomosis and it appears to be associated with regions of separation and low wall shear. This work improves understanding of one of the mechanisms which causes delayed failure of synthetic grafts in the arterial system.